Progress has been made in engineering plant cells to produce useful proteins. For example, plants have been shown to express potentially medically important proteins that may be used for immunization against pathogens. Many infectious diseases require booster vaccinations or multiple antigens to induce and maintain protective immunity. Advantages of plant-derived vaccines include the delivery of multiple antigens, low cost of production, storage & transportation, elimination of medical personnel and sterile injections, heat stability, antigen protection through bioencapsulation, the generation of systemic & mucosal immunity and improved safety via the use of a subunit vaccine and absence of human pathogens. Despite cases of successful expression of proteins, the development of plant derived medically important compositions is still in its formative stages.